Rust Impl Cross Compile

rust-impl-cross-compile

The cross-compilation skill: how to build a Rust crate for a target other than the host. Covers target triple format and common triples, rustup target add to install standard library artifacts, cargo build --target <triple>, the cross crate for container-based toolchains, conditional compilation via cfg(target_os = ...) and related predicates, the cfg_attr attribute for selective attribute emission, and per-target linker / runner configuration via .cargo/config.toml.

Cross-references: [[rust-impl-cargo-project]] (single-crate Cargo.toml + profiles), [[rust-impl-build-scripts]] (build.rs that detects target via env vars), [[rust-impl-ffi-bindgen]] (cross-compile C deps and adjust sysroot), [[rust-core-toolchain]] (rustup channels, components, target lists).

When to use this skill

• User runs cargo build and gets linker 'cc' not found or error: target may not be installed after switching --target.
• User wants a portable static Linux binary and asks musl vs gnu.
• User builds for Windows from Linux and asks pc-windows-msvc vs pc-windows-gnu.
• User builds for macOS ARM (Apple Silicon) from x86_64 host.
• User asks how to build a .wasm module and asks wasm32-unknown-unknown vs wasm32-wasip1.
• User writes platform-conditional code with #[cfg(target_os = "...")] or cfg!(...).
• User wants a single attribute (e.g. #[repr(C)], #[link(...)]) to appear only on one target.
• User configures a custom linker for a target via [target.<triple>] linker = "..." in .cargo/config.toml.
• User asks about the cross tool because installing native toolchains is tedious.

For host-only profile / lints / dependency tuning see [[rust-impl-cargo-project]]. For build-time target detection in build.rs see [[rust-impl-build-scripts]].

Target triple format

Rust target triples follow the shape <arch>-<vendor>-<os>-<env> where each field is a fixed identifier (see platform-support):

ALWAYS write the full triple. NEVER abbreviate to linux or windows. NEVER assume a host target; query with rustc -vV | grep host.

x86_64-unknown-linux-gnu      arch=x86_64  vendor=unknown  os=linux    env=gnu
x86_64-unknown-linux-musl     arch=x86_64  vendor=unknown  os=linux    env=musl
aarch64-apple-darwin          arch=aarch64 vendor=apple    os=darwin   env=(none)
x86_64-pc-windows-msvc        arch=x86_64  vendor=pc       os=windows  env=msvc
x86_64-pc-windows-gnu         arch=x86_64  vendor=pc       os=windows  env=gnu
wasm32-unknown-unknown        arch=wasm32  vendor=unknown  os=unknown  env=(none)
wasm32-wasip1                 arch=wasm32  vendor=unknown  os=wasi     env=p1
thumbv7em-none-eabihf         arch=thumbv7em vendor=none   os=none     env=eabihf

NOTE: wasm32-wasi was renamed to wasm32-wasip1 in Rust 1.81 (rust181). wasm32-wasi still works as a legacy alias but new code should use wasm32-wasip1 (or wasm32-wasip2 for WASIp2).

Common target triples

Decision rules per use case. Tier classification is from platform-support: Tier 1 has CI guarantees + host tools, Tier 2 with-host-tools has guaranteed std + tools, Tier 2 without-host-tools has guaranteed std only, Tier 3 is best-effort.

Linux: gnu vs musl

Need a portable static binary that runs on any Linux distro (no glibc dep)?
   YES -> *-unknown-linux-musl
   NO  -> *-unknown-linux-gnu (default, dynamic glibc, smaller binary, glibc 2.17+)

Will the binary load shared libraries with dlopen at runtime?
   YES -> *-gnu (musl has known dlopen quirks)
   NO  -> either is fine, choose by static vs dynamic preference

Is the binary going into a `FROM scratch` Docker image?
   YES -> *-musl is the only sane choice

Windows: msvc vs gnu

Compiling on Windows with Visual Studio Build Tools installed?
   YES -> *-pc-windows-msvc (recommended, matches Microsoft ecosystem)

Cross-compiling from Linux / macOS to Windows?
   *-pc-windows-gnu is easier (just install mingw-w64 cross gcc).
   *-pc-windows-msvc also works via `cargo-xwin` or LLVM tooling but heavier setup.

Linking against a precompiled `.lib` from a vendor?
   Match the vendor's ABI. MSVC libs require msvc; MinGW libs require gnu.

WebAssembly: unknown vs wasi

Browser target with JS interop?  -> wasm32-unknown-unknown + wasm-bindgen.
CLI / serverless WASI runtime (wasmtime, wasmer)?  -> wasm32-wasip1 (or wasip2 for component model).
Pure compute, no I/O, no DOM?    -> wasm32-unknown-unknown (smaller, no OS shims).

Cross-compile workflow

Step 1 : install the target standard library

rustup target add x86_64-unknown-linux-musl

rustup target add installs ONLY the precompiled standard library for the target (rustup-cross). It does NOT install a linker, libc, or any platform SDK. The host toolchain (compiler) is reused; only libstd / libcore / liballoc are added.

List available targets : rustup target list. List installed : rustup target list --installed. Remove : rustup target remove <triple>.

Step 2 : ensure a target-compatible linker exists

NEVER expect rustup target add to install gcc / clang / lld. ALWAYS install the host-side cross toolchain separately, or delegate to cross.

Step 3 : point Cargo at the linker via .cargo/config.toml

# .cargo/config.toml (project-local or in $CARGO_HOME)
[target.aarch64-unknown-linux-gnu]
linker = "aarch64-linux-gnu-gcc"

[target.x86_64-unknown-linux-musl]
linker = "musl-gcc"

[target.x86_64-pc-windows-gnu]
linker = "x86_64-w64-mingw32-gcc"

[target.<triple>] keys (cargo-config) :

Triple names in env vars uppercase and replace dashes with underscores : CARGO_TARGET_X86_64_UNKNOWN_LINUX_GNU_LINKER.

Step 4 : build

cargo build --target x86_64-unknown-linux-musl --release
# Binary in target/x86_64-unknown-linux-musl/release/<name>

ALWAYS include --release for distribution builds. The target dir is nested under target/<triple>/, so cross builds never overwrite host builds.

Pin a default target for a project

# .cargo/config.toml
[build]
target = "x86_64-unknown-linux-musl"

Now bare cargo build cross-builds without --target. NEVER do this in a library crate (forces all downstream users to install that target).

The cross tool (container-based)

cross (cross-rs) is a community wrapper that runs cargo inside a per-target Docker / Podman image. Each image bundles the cross linker + libc + sysroot, so the host needs only Docker.

cargo install cross --git https://github.com/cross-rs/cross
cross build --target aarch64-unknown-linux-gnu --release

Use cross when :

• Host toolchain setup is awkward (musl-cross on Debian, mingw-w64 on macOS, etc.).
• You build many targets in CI and want hermetic, reproducible toolchains.
• You want Windows / Android / BSD targets without installing each SDK.

NEVER use cross for aarch64-apple-darwin from a non-macOS host : Apple's SDK license forbids redistribution, so no Docker image exists. ALWAYS build Darwin targets on macOS hardware or via macOS CI runners.

cross reads Cross.toml for per-target image overrides, mounted volumes, and pre-build hooks. See [[references/methods.md]] for syntax.

Conditional compilation with cfg

cfg predicates select code that is compiled only when the predicate holds (reference-cfg). All cfg keys are evaluated at compile time against the current target.

Common cfg keys

#[cfg(target_os = "linux")]
fn platform() -> &'static str { "linux" }

#[cfg(target_os = "macos")]
fn platform() -> &'static str { "macos" }

#[cfg(target_os = "windows")]
fn platform() -> &'static str { "windows" }

Combining predicates

#[cfg(all(target_os = "linux", target_arch = "x86_64"))]
fn fast_linux_x64() {}

#[cfg(any(target_os = "linux", target_os = "macos"))]
fn unix_like() {}

#[cfg(not(target_family = "windows"))]
fn not_windows() {}

Use all(...), any(...), not(...) for boolean composition. NEVER nest cfg attributes (#[cfg(a)] #[cfg(b)]) when you mean all ; while it works, cfg(all(a, b)) makes intent explicit.

Runtime cfg!(...) macro

if cfg!(target_os = "linux") {
    // executed only on Linux. Code is still type-checked on every target.
}

cfg!(...) returns a bool constant. Both arms of the if are compiled on every target, so they must type-check everywhere. For mutually exclusive code, prefer #[cfg(...)] on items.

cfg_attr for conditional attributes

#[cfg_attr(predicate, attr1, attr2, ...)] emits the listed attributes ONLY when the predicate is true ([reference-cfg]).

#[cfg_attr(target_os = "linux", repr(C))]
struct Header {
    magic: u32,
    flags: u32,
}

// On Linux, expands to: #[repr(C)] struct Header { ... }
// On other targets, no repr attribute is applied.

#[cfg_attr(not(test), inline)]
fn hot_path() { /* ... */ }

ALWAYS use cfg_attr when an attribute itself should be conditional. NEVER copy-paste the whole item under #[cfg(...)] just to vary one attribute.

Quick-start workflow recipes

Static Linux binary from Linux host

sudo apt install musl-tools                  # provides musl-gcc
rustup target add x86_64-unknown-linux-musl
# Add to .cargo/config.toml: [target.x86_64-unknown-linux-musl] linker = "musl-gcc"
cargo build --release --target x86_64-unknown-linux-musl
file target/x86_64-unknown-linux-musl/release/<name>     # statically linked

Windows .exe from Linux host (MinGW)

sudo apt install gcc-mingw-w64-x86-64
rustup target add x86_64-pc-windows-gnu
# Add to .cargo/config.toml: [target.x86_64-pc-windows-gnu] linker = "x86_64-w64-mingw32-gcc"
cargo build --release --target x86_64-pc-windows-gnu

WebAssembly (WASI) from any host

rustup target add wasm32-wasip1
cargo build --release --target wasm32-wasip1
wasmtime target/wasm32-wasip1/release/<name>.wasm

Many targets with cross

cargo install cross --git https://github.com/cross-rs/cross
cross build --release --target aarch64-unknown-linux-gnu
cross build --release --target armv7-unknown-linux-gnueabihf
cross build --release --target x86_64-pc-windows-gnu

Validation checklist

Before claiming a cross-build is done :

1. rustup target list --installed | grep <triple> shows the target.
2. Linker is on PATH or pinned in .cargo/config.toml.
3. cargo build --target <triple> exits 0.
4. file target/<triple>/release/<bin> reports the expected ELF / PE / Wasm format.
5. For musl : ldd target/<triple>/release/<bin> reports not a dynamic executable.
6. For Wasm : wasm-objdump -h <bin>.wasm lists sections without errors.
7. Platform-conditional code paths were exercised on the actual target (or under an emulator) ; cfg! only verifies the predicate, not the runtime correctness.

Reference links

• [[references/methods.md]] : full rustup / cargo / cross command catalogue, all [target.<triple>] keys, full cfg-key list with valid values, env-var equivalents.
• [[references/examples.md]] : end-to-end recipes (musl static, Windows MinGW from Linux, macOS ARM, WASI, Cortex-M no_std, Cross.toml, GitHub Actions matrix).
• [[references/anti-patterns.md]] : missing rustup target add, choosing musl for the wrong reason, wasm32-unknown-unknown for WASI workloads, hard-coding linker path, missing cfg_attr, untested conditional code paths.